Database Migrations

Migrations are a convenient way to alter your database schema over time in a consistent and easy way. Traditionally, you have to write raw SQL to modify the database schema. In Rails, you only need to write Ruby code to do the migrations.

You can think of each migration as being a new version of the database. A schema starts off with nothing in it, and each migration modifies it to add or remove tables, attributes and other modifications. Active Record tracks your schema with a history timeline, enabling it to go back and forth to any versions of your schema. Active Record will update your db/schema.rb file to match the up-to-date version of your database schema.

Data Types

In a relational database, such as PostgreSQL and SQLite3, you have many data type to choose from for your attributes. Primarily, we have data types that are designed to store numbers, text and dates.

Below is a list of commonly used data types for database attributes:

Using the change Method

To create a migration, use $ rails generate migration <Name of Migration>. To add a column author_id to the books table, we can use $ rails generate migration AddAuthorIdToBooks

Make sure your migration file name makes semantic sense. For example, if you are trying to add first_name to the students table, name it AddFirstNameToStudents.

After you create an empty migration, you can use a lot of different methods to modify the database schema.

Here's a list of methods you can use:

For example, to add first_name with string data type to books table, you can do:

class AddFirstNameToStudents < ActiveRecord::Migration[5.0]
  def change
    add_column :books, :first_name, :string
  end
end

To index the attribute first_name, you can do

What's indexing? Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed

...
  def change
    add_index :books, :first_name
  end
...

Rollback

Migration's job is to modify the current table schema. Another common task is to rollback the last migration (to revert the changes). For example, if you makes a mistake in your migration file and intend to fix it, you can rollback / revert the previous migration using $ rails db:rollback. You can also track down the exact version number associated with the previous migration.

$ rails db:migrate:status

Status   Migration ID    Migration Name
--------------------------------------------------
  up     20161212101030  Create books

$ rails db:rollback

$ rails db:migrate:status

Status   Migration ID    Migration Name
--------------------------------------------------
  down     20161212101030  Create books

You can see that the status went from "up" to "down" after we rollback the previous migration.

Remember, when you run $ rails db:rollback, it will only rollback the previous ONE migration. But, when you run $ rails db:migrate, it will migrate ALL the migrations that aren't "up" yet.

What if you want to rollback more steps at once? $ rails db:rollback STEP=3 will revert the last 3 migrations.

What if you want to rollback to an exact version? $ rails db:rollback VERSION=x will revert the exact migration specified by the version number.

References:

1. ActiveRecord Migrations